Spontaneous Separation of Sand Grains

In 1867, Lord Kelvin described an experiment in which two streams of water droplets were connected so that each stream amplified the charge on the second stream [W. Thomson, Proc. R. Soc. London 16, 67 (1867).].

Amit Mehrotra, Fernando J. Muzzio, and Troy Shinbrot of the Department of Chemical & Biochemical Engineering, Rutgers University, present here a complementary effect in flowing grains that spontaneously separates similar and well-mixed grains into two charged streams of demixed grains.

The grains are positively charged. While they sit on a surface, the mutual repulsion of the grains can propel some of them as high as 2 meters into the air. This effect of spontaneous separation has important consequences for industrial and natural processes.